RESEARCHES

In this study, we introduce a novel concept of real-time microscopic particle image velocimetry (micro-PIV) for apparent high-speed microchannel flows in a lab-on-a-chip by using a frame-straddling high-speed vision system with two camera inputs; it can synchronize two camera inputs with the same view field with a time delay on a submicrosecond time scale. To improve the measurable range of velocity in microchannel flow observation, we propose a variable-frame-straddling optical flow (VFS-OF) algorithm that can simultaneously estimate the microchannel flow distribution as gradient-based optical flows using frame-straddled images from the two camera inputs; their frame-straddling time is adaptively determined by the amplitude of the estimated optical flow to avoid large image displacements between frames that often generate serious errors in optical flow estimation.



We built a real-time micro-PIV system by software-implementing the VFS-OF algorithm in a high-speed vision system with twoframe-straddled cameras; it can execute real-time video processing and recording of 512×512-pixel images at 2000 frames per second for the two cameras and control their frame-straddling time from 0 to 0.5ms with 9.9-ns steps.




Our real-time micro-PIV system can estimate the velocity distribution of apparent high-speed microchannel flows at 1m/s or more in real time by controlling the frame-straddling time. Experimental results were performed for microfluidic flows on microchannels with widths of hundreds of micrometersto verify the performance of our real-time micro-PIV system based on the VFS-OF algorithm.